This invention relates to an inverter control apparatus, and more particularly to an apparatus suitable for controlling an inverter which converts a d.c. output voltage of an a.c./d.c. converter into a variable a.c. voltage having a variable frequency.
Japanese Patent Publication No. 61-48367 (1986) is known as one of prior art publications disclosing a control method of this kind. Japanese Patent Publication No. 61-48356 notices the fact that, when an a.c./d.c. converter converts an a.c. voltage into a d.c. voltage, and the d.c. output voltage of the converter is applied to a pulse width modulation inverter to be converted into a variable a.c. voltage having a variable frequency (VVVF), the output voltage of the inverter pulsates, and, especially, a beat phenomenon occurs at a specific output frequency of the inverter, because the output voltage of the a.c./d.c. converter, that is, the input voltage of the inverter includes a pulsating component (rectification ripples which occurred during rectification). According to the method of disclosed in Japanese Patent Publication No. 61- 48356 which solves the above problem, the ratio between the amplitude of a sine wave signal and that of a carrier signal of triangular waveform, that is, the pulse width of a PWM signal is adjusted to deal with a variation of the inverter input voltage, so that the inverter output voltage can be freed from any variation.
Also, Japanese Patent Application No. 57-52383 proposed to attain the same object discloses a method of controlling a PWM inverter in which a pulse processing technique is used so as to adjust the pulse width of the PWM signal to deal with a variation of the inverter input voltage.
However, these prior art control methods have such a disadvantage that the desired control is not applicable to a voltage range in which the output voltage of the inverter attains its maximum level and any further voltage control is impossible. That is, the desired control is not applicable because the number of pulses included in one cycle of the inverter output voltage is only one, and the inverter output voltage is maintained constant at its maximum level.
Also, electric railway vehicles, as will be later described, receive a single-phase a.c. voltage from an aerial line. If this single-phase a.c. voltage is converted to a d.c. voltage by a converter, a large rectification ripple is present particularly at a frequency double the frequency of the single-phase a.c. voltage, thereby aggravating a beat phenomenon.